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22
CHAPTF,FDESCENTWITH MODIFICATION:
A DARWINIAN VIEW OF LIFE
OUTLII\'E
Historical{ontext for Evolutionary Theory
A. Western culture resistedevolutionary views of life
B. Theories of geological gradualismhelped clear the path for evolutionary biologists
l.
II.
C. Lamarck placed fossils in an evolutionary context
The Darwinian Revolution
A. Field researchhelped Darwin frame his view of life'. scienceas a process
B. The Origin of Speciesdevelopedtwo main points: the occurrence of evolution and
natural selectionas its mechanism
III. Evidence of Evolution
A. Evidenceof evolution pervadesbiology
B, What is theoretical about the Darwinian view of life?
OBJECTTVES
After reading this chapter and attending lecture,the student should be able to:
State the two major points Darwin made in The Origin of Species concerning the
1.
E,arth'sbiota.
Z.
Compareand contrastPlato'sphilosophyof idealismand Aristotle'sscala naturae.
DescribeCarolusLinnaeus'contributionto Darwin'stheory of evolution.
3.
4.
DescribeGeorgesCuvier'scontributionto paleontology'
Explain how Cuvier and his followers usedthe concept of catastrophismto oppose
5.
evolution.
Explain how the principle of gradualism and Charles Lyell's theory of
uniformitarianism influencedDarwin's ideas about evolution.
Describe Jean Baptiste Lamarck's model for how adaptationsevolve.
7.
Describehow CharlesDarwin usedhis observationsfrom the voyage of the HMS Beagle
8.
to formulate and support his theory of evolution'
g.
Describehow Alfred Russelwallace influencedcharles Darwin.
"descent with
10. Explain what Darwin meant by the principle of common descentand
modification".
1 1. Explain what evidenceconvincedDarwin that specieschangeover time.
12. State, in their own words, three inferences Darwin made from his observations, which
led him to proposenatural selectionas mechanismfor evolutionarychange.
6.
13.
14.
15.
16.
Explain why variation was so importantto Darwin's theory.
Explain how ReverendThomasMalthus'essayinfluencedCharlesDarwin.
Distinguishbetweenartificial selectionand naturalselection.
Explain why the populationis the smallestunit that can evolve.
326
Unit IV Mechanisms
of Evolution
11.
Using some contemporary examples, explain how natural selection results in
evolutionary change.
Explain why the emergenceof populationgeneticswas an important turning point for
evolutionary theory.
Describethe lines of evidenceCharlesDarwin usedto supportthe principle of common
descent.
18.
19.
20.
21.
22.
Describehow molecular biology can be usedto study the evolutionary relationships
among organisms.
Explain the problem with the statementthat Darwinism is 'Just a theory".
Distinguishbetweenthe scientificand colloquialuse of the word "theory".
KEYTBRMS
evolution
fossils
natural selection
descentwith modification
artificial selection
sedimentaryrocks
evolutionary adaptations paleontology
natural theology
gradualism
biogeography
homology
taxonomy
homologous+tructures
uniformitarianism
vestigialorgans
ontogeny
phylogeny
LECTT]RENOTBS
Evolution, the unifoing theme woven throughout the text and course, refers to the processesthat
have transformed life on earth from its earliest forms to the enorrnous diversity that
characterizes it today.
The first convincing case for evolution was published in a book by Charles Darwin on November
24,1859.In this book, On the Origin of Speciesby Means of Natural Selection,Darwin:
'
Synthesized seemingly unrelated facts into a conceptual framework that accounts for
both the unity and diversity of life.
.
Discussedimportant biological issuesabout organisms, such as why there are so many
kinds of organisms,their origins and relationships,similaritiesand differences,geographic
distribution, and adaptationsto their environment.
.
Made two major points:
1. Speciesevolved from ancestralspeciesand were not specially created.
2. Natural selection is a mechanismthat could result in this evolutionary change.
I.
Historical Context for Evolutionary Theory
A. Western culture resisted evolutionary views of life
The impact of Darwin's ideaspartially dependedupon historicaland social context(see
Campbell,Figure 22.1).
.
Darwin's view of life contrastedsharply with the acceptedviewpoint: the Earth
was only a few thousand years old and was populated by unchanging life forms
made by the Creator during a single week.
.
Thus, On the Origin of Species by Means of Natural Selection not only
challengedprevailing scientific views, but also challengedthe roots of Western
culture.
Chapter22 DescentWith Modification:A DarwinianYiew of Life 327
t . The scale of nature and natural theology
Many Greek philosophersbelieved in the gradual evolution_of life. However, the
two ihat influenced Western culture most, Plato (427 347 B-C.) and his student
inconsistentwith a concept
were
which
Aristotle (384 3228.C.), held opinions
of evolution.
plato, whose philosophy is known as idealism (essentialism),believedthat
.
there were two coeiisting worlds: an ideal, eternal, real world and an
illusionary imperfect *orid that humans perceive with their senses' T o
Plato,
+ Variations in plant and animal populations were merely imperfect
representativesof ideal forms; only the perfect ideal forms were real.
Evolution would be counterproductive in a world where ideal organisms
were already perfectly adaptedto their environments'
Aristotle questionedthe Platonic philosophy of dual worlds, but his beliefs
also excludedevolution.
=+ Recognizing that organismsvary from simple to complex, he believed
that they could be placed on a scale of increasing complexity (scala
naturae); on this ladder of life, each form had its allotted rung and each
mng was occuPied'
= In this view of life, specieswere fixed and did not evolve'
= The scala naturae view of life prevailed for over 2000 years.
+
.
The creationist-essentialistdogmathat specieswere individually createdand fixed
became embeddedin Western thought as the Old Testamentaccount of creation
from the Judeo-Christianculture fortified prejudice against evolution.
. Natural Theology,a philosophy that the Creator'splan could be revealed by
studying nature-, dominated European and American biology even as
Darwinism emerged.
.
For natural theologians, adaptations of organisms were evidence that the
Creator had designedevery speciesfor a particular purpose'
. Natural theology's major objective was to classiff speciesrevealing God's
createdstepson the ladder of life.
carolus Linnaeus (1707 - 1778),a Swedishphysician and botanist, sought order in
the diversity of life ad maiorem Dei gloriam (for the greater glory of God).
naming and classifying of
.
Known as the father of taxonomy-the
nomenclature still used
binomial
of
system
organisms-he developedthe
todaY.
He adopted a system for grouping speciesinto categories and ranking the
categoriesinto a hierarchy. For example, similar speciesare grouped into a
g"nus; similar generaare groupedinto the sameorder'
Linnaeus found order in the diversity of life with his hierarchy of taxonomic
categories.
.
The clustering of speciesin taxonomic groups did not imply evolutionary
relationships to Linnaeus, since he believed that specieswere permanent
creations.
.
Linnaeus, a natural theologian, developedhis classification scheme only to
reveal God'splan and even stated Deus creavit, Linnaeus disposuit ("God
creates,Linnaeus arranges")'
.
328
Unit IV Mechanisms
of Evolution
2. Cuvier, fossils, and catastrophism
Fossils : Relics or impressionsof organismsfrom the past preservedin rock
.
Most fossils are found in sedimentaryrocks, which:
Form when new layers of sand and mud settle to the bottom of seas,
lakes, and marshes, covering and compressing older layers into rock
(e.g. sandstoneand shale)
= May be deposited in many layers (strata) in places where shorelines
repeatedlyadvanceand retreat. Later erosion can wear away the upper
(younger) strata,revealing older stratawhich had been buried.
'
The fossil record thus provides evidence that Earth has had a successionof
flora and fauna(seeCampbell,Figure 22.2).
The study of fossils,paleontology, was founded by the French anatomistGeorges
Cuvier (1769-1832)who:
.
Realized life's history was recorded in fossil-containing strata and
documentedthe succession
of fossil speciesin the ParisBasin
'
Noted each stratum was characterizedby a unique set of fossil species and
that the older (deeper) the stratum, the more dissimilar the flora and fauna
from modern life forms
.
Understoodthat extinction had been a common occurrencein the history
of life since, from stratum to stratum, new species appeared and others
disappeared
Even with paleontological evidence,Cuvier was an effective opponent to the
evolutionistsof his day.
.
He reconciled the fossil evidencewith his belief in the fixity of speciesby
speculating that boundariesbetween fossil strata correspondedin time to
catastrophicevents, such as floods or droughts.
.
This view of Earth's history is known as catastrophism.
Catastrophism : Theory that major changesin the Earth's crust are the result of
catastrophicevents rather than from gradual processesbf change
Cuvier explained the appearanceof new speciesin younger rock that were absent
from older rock by proposing that:
.
Periodic localizedcatastrophes
resultedin massextinctions.
. After the local flora and fauna had become extinct, the region would be
repopulatedby foreign speciesimmigrating from other areas.
=
B. Theories of geological gradualism helped clear the path for evolutionary
biologists
In the late l8th century, a new theory of geological gradualism gained popularity
among geologiststhat would greatly influence Darwin.
Gradualism: Principle that profound changeis the cumulative product of slow,
continuousprocesses
.
Competed with Cuvier's theory of catastrophism
'
Proposedby JamesHutton (1975), a Scottish geologist. He proposedthat it was
possible to explain the various land forms by looking at mechanismscurrently
operating in the world.
Example: Canyons form by erosion from rivers, and fossil-bearingsedimentary
rocks form from particles eroded from the land and carried by rivers to the sea.
with Modification:A DarwinianView of Life 329
chapter22 Descent
gradualism into
Charles Lyell, a leading geologistof Darwin's time, expandedHutton's
the theory known as uniformitarianism'
and have operated
Uniformitarianism : Theory that geological processesare uniform
from the origin of the Earth to the present
that their
.
It was Lyell's extreme idea that geologicalprocessesare so uniform
rates and effects must balanceout through time'
the erosion
.
Example:processesthat build mountainsare eventuallybalancedby
of mountains'
that
Darwin rejecteduniformitarianism,but was greatly influencedby conclusions
Lyell:
and
Hutton
of
followed directly from the observations
slow, gradual
.
The Earth must be ancient. If geological change results from
older than
much
processesrather than sudden"u.it., then the Earth must be
inference'
the 6000 years indicatedby many theologianson the basisof biblical
. Very slow and subtle processespersisting over a great length of time can cause
substantialchange.
C. Lamarck placed fossils in an evolutionary context
along with Earth's
Several18th centurynaturalistssuggestedthat life had evolved
(1809) a
and
LamaiJt
0744-1829)_deve.l.oped published
changes.Only Jean'eaptiste
comp"rehensivemodelwhichattemptedtoexplainhowlifeevolved.
History Museum in
Lamarck was in charge of the invertebratecollection at the Natural
Paris,which allowed him to:
identiff several
.
Compare modern speciesto fossil forms, and in the process,
to younger
fossils
older
of
series
lines of descentcomposedof a chronological
fossilsto modern sPecies'
(as opposed to
.
Envision many laddersof life which organismscould climb
movement)'
Aristotle's single ladderwithout
which were
= The bottom rungs were occupied by microscopic organisms
continuallygeneratedspontaneouslyfromnonlivingmaterial.
+ At the tops of the ladderswere the most complex plants and animals'
toward increasing
Lamarck believed that evolution was driven by an innate tendency
complexity, which he equatedwith perfection'
adapted to
.
As organisms attained perfection, they became better and better
their environments.
sentiments
.
Thus, Lamarck believed that evolution respondedto organisms'
interieurs ("felt needs").
which included
Lamarck proposeda mechanismby which specific adaptationsevolve,
fwo related princiPles:
cope with the
l. (Jse and disuse. Those body organs used extensively to deteriorate'
environment become larger an-dstrongerwhile those not used
organism acquired
2. Inheritance of acquired characteristics.The modifications an
during its tifetimJcould be passedalong to its offspring'
credit for
Although his mechanismof evolution was in error, Lamarck deserves
proposing that:
the extant
.
Evolution is the best explanation for both the fossil record and
diversitY of life.
.
The Earth is ancient'
330
Unit IV Mechanisms
of Evolution
.
tr.
Adaptation to the environmentis a primary product of evolution.
The Darwinian Revolution
At the beginning of the l9th century, natural theology still dominated the European
and American intellectualclimate.In 1809,the sameyear Lamarck publishedhis
theory of evolution, CharlesDarwin was born in Shrewsbury,England.
'
Though interestedin nature,Charles(at l6) was sent by his physician father to
the University of Edinburghto study medicine, which he found boring and
distasteful.
'
He left Edinburgh without a degree and enrolled at Christ College, Cambridge
University to prepare for the clergy.
+ Nearly all naturalists and other scientists were clergymen, and a majority
held to the philosophyof natural theology.
=+ Charles studied under the Reverend John Henslow, a botany professor at
Cambridge,and receivedhis B.A. degreein 1831.
= Professor Henslow recommendedhim to Captain Robert FitzRoy who was
preparing the survey ship HMS Beagle for an around the world voyage.
A. Field research helped Darwin frame his view of life: science ils a process
l. The voyage of the Beagle
The HMS Beagle, with Darwin aboard,sailed from England in December 1831 (see
CampbellFigure 22.3).
'
The voyage's mission was to chart the poorly known South American
coastline.
'
While the ship'screw surveyedthe coast, Darwin spent most of his time
ashorecollecting specimensof the exotic and diverse flora and fauna.
While the ship worked its way around the continent, Darwin observedthe various
adaptationsof plants and animalsthat inhabitedthe diverseenvironmentsof South
America: Brazilianjungles, grasslands
of the Argentinepampas,desolateislandsof
Tierra del Fuego, and the Andes Mountains. Darwin noted the following:
'
The South American flora and fauna from different regions were distinct
from the flora and fauna of Europe.
'
Temperate specieswere taxonomically closer to speciesliving in tropical
regionsof South America than to temperatespeciesof Europe.
'
The South American fossils he found (while differing from modern species)
were distinctly SouthAmerican in their resemblanceto the living plants and
animals of that continent.
Geographicaldistribution was particularly confusing in the caseof the fauna of the
Galapagos,recentlyformed volcanic islandswhich lie on the equatorabout 900 km
west of South America.
Most animal specieson the Galapagosare unique to those islands, but
resemblespeciesliving on the SouthAmericanmainland.
Darwin collected l3 types of finches from the Galapagos,and although they
were similar, they seemedto be different species.
=+ Somewere uniqueto individual islands
=+ Others were found on two or more islandsthat were close tosether
Chapter22 DescentWith Modification:A DarwinianView of Life 331
,,
By the time the Beagle left the Galapagos,Darwin had read Lyell's Principles of
Geology,and was influencedby Lyell's ideas.
.
Darwin had begunto doubt the church's position that the Earth was static
and had been cieated only a few thousandyears before'
.
when Darwin acknowledgedthat the Earth was ancient and constantly
changing, he had taken an important step toward recognizing that life on
Earth had also evolved.
Darwin focuses on adaPtation
Darwin was not sure whether the l3 types of finches he collected on the Galapagos
were different speciesor varietiesof the samespecies'
.
After he returnedto Englandin 1836, an ornithologist indicatedthat they
were actuallYdifferent sPecies.
observationsmade during the voyage and in 1837 began the
He reassessed
first notebookon the origin of species'
Darwin perceivedthe origin of 1ew^speciesand adaptationas closely related
processes;new speciescould arise from an ancestralpopulation by gradually
accrmulaiing adaptationsto a different environment. For example,
. Two populationsof a speciescould be isolated in different environments
and divergeas eachadaptedto local conditions'
.
Over many generations,the two populations could become dissimilar
enoughto be designatedseparatespecies'
.
This is apparentlywhat happenedto the Galapagosfinches; their different
(See
beaksare adaptationsto speilfic foods availableon their home islands.
.
Campbell,Figure22.4)
By the early 1840s,Darwin had formed his theory of natural selection as the
mechanism-ofadaptiveevolution,but delayedpublishingit'
.
Reclusiveand in poor health, Darwin was well known as a naturalist from
the specimers un^dletters he had sent to Britain from the voyage on the
Beagle.
.
He frequently correspondedand met with Lyell, Henslow, and other
scientists.
In 1844,Darwin wrote a long essayon the origin of speciesand natural selection'
.
He realizedthe importance and subversivenature of his work, but did not
in
nublish the information becausehe wished to gather more evidence
iupport of his theorY.
.
Evolutionary thinking was emerging at this time, and Lyell ad-monished
Darwin to pubtisn on ih. subjectbefore someoneelse published it first.
as a
In June 1858, Darwin receiveda letter from Alfred Wailace, who was working
specimencollector in the East Indies.
.
Accompanying the letter was a manuscriptdetailing Wallace'sown theory
of natuial ieleition which was almost identical to Darwin's.
.
The letter asked Darwin to evaluatethe theory and forward the manuscript
to Lyell if it was thoughtworthy of publication'
"smashed."
.
Darwin did so, althoughhe felt that his own originality would be
.
Lyell and a colleaguepresentedWallace'spaper along with excerpts from
Darwin's unpublisliedi844 .rruy to the LinnaeanSocietyof London on July
1, I 858.
332
Unit IV Mechanisms
of Evolution
Darw
n
#*,:',l: z:;,:f:;
;!,::ff : T*fJl':T
:iJ"H r: lT: deve,opedan
d
supportednatural selection
much more extensivelythan wallace.
Darwin's book and its proponents quickly convinced the majority of
biologiststhat biodiversityis a product of evolution.
Darwin succeededwhere previous evolutionists had failed not only because
sciencewas moving away from natural theology, but becausehe ionvinced
his readerswith logic and evidence.
B. The Origin of Speciesdeveloped two main points: the occurrence of evolution
and natural selection as its mechanism
l. Descent with modification
Darwin used the phrase"descentwith modification,"not evolution.in the first
edition';:::::,::#
^(Y;';'r;ure with an organisms
rerated
throughdescent
rrom
some unknown ancestralpopulationthat lived in the remote past.
Diverse modifications (adaptations) accumulatedover millions of years, as
descendantsfrom this common ancestormoved into various habitais.
Darwin's metaphor for the history of life was a branching tree with multiple
branchingfrom a common trunk to the tips of living twigs, symbolic of the
o'""j"'"oi1:ilt?T.?"Tl#il1T;,
isanancestrar
popuration
common
to ail
evolutionary lines of descentbranching from that fork.
Speciesthat are very similar sharea common ancestorat a recent branch
point on the phylogenetictree.
Less closely relatedorganismssharea more ancientcommon ancestorat an
earlier branch point.
Most branchesof evolution are dead ends since abofi 99yo of all species
that ever lived are extinct.
To Darwin, Linnaeus' taxonomicschemereflectedthe branchinggenealogyof the
tree of life.
'
It recognized that the diversity of organismscould be ordered into "groups
-levels
subordinateto groups", with organisms at the different taxonomic
relatedthrough descentfrom common ancestors.
'
Classificationalone does not confirm the principle of common descent,but
when combinedwith other lines of evidence,the relationshipsare clear.
'
For example, genetic analysis of species that are thought to be closely
related on the basis of anatomical features and other criteria reveals a
common hereditary background.
2. Natural selection and adaptation
Darwin's book focusedon the role of naturalselectionin adaptation(seeCampbell,
Figure 22.5). Ernst Mayr of HarvardUniversitydissectedthe logic of Darwin'i
theory into three inferencesbasedon five observations:
'
Observation.l: All specieshave such great potential fertility that their
populationsizewould increaseexponentially if all individualstLat are born
reproducedsuccessfully.
'
Observation 2: Populations tend to remain stable in size, except for
seasonalfluctuations.
With Modification:A DarwinianView of Life 333
Chaoter22 Descent
.
.
.
Environmentalresourcesare limited'
Observation3:
can
.
Inference.l: Production of more individualsthan the environment
a
of
individuals
among
existence
sJpport leads to a struggle for
population,withonlyafractionofoffspringsurvivingeachgeneration.
in their
Observation4: Individuals of a population vary extensively
alike'
exactly
are
no two individuals
characteristics;
Observation5'. Much of this variation is heritable'
. Inference 2: Survival in the struggle for existence is not random, but
dJpends in part on the herediiary constitution of the surviving
individuals.Tiose individualswhose inherited characteristicsfit them
less
best to their environment are likely to leave more offspring than
fit individuals.
. Inference J: This unequalability of individualsto survive and reproduce
will lead to a gradual change in a population, with favorable
accumulatingover the generations'
characteristics
SummarizingDarwin's ideas:
. Natural selectionis this differential successin reproduction, and its product
is adaptationof organismsto their environment'
. Natural selectionoccurs from the interaction between the environment and
the inherent variability in a population'
. Variations in a population arise by chance,-butnatural selection is not a
for
chance ph.noln"non, since environmental factors set definite criteria
reproductivesuccess.
Darwin was already aware of the strugglefor existencecausedby overproduction'
Thomas
when he read an essayon human population written by the Reverend
Malthus (1798).
. In this esEay,Malthus held that much of human suffering was a consequence
of human populationsgrowing faster than the food supply'
only a
.
This capacity for overproduction is common to all species, and
of
offspring
leave
and
fraction of new individuals complete development
their own; the rest die or are unable to reproduce'
possible.
Variation and overproduction in populationsmake natural selection
genes on to more
.
On the average, the most fit individuals pass their
offspring than less fit individuals'
over
.
This results from environmentalediting, which favors some variations
others.
that natural selection
From his experienceswith artificial selection,Darwin inferred
could causeiubstantial changein populations'
. Through the breeding of domesticatedplants and animals, humans have
with desired
modified speciesover many generationsby selectingindividuals
traits as breeding stock.
to their
.
The plants and animals we grow for food show little resemblance
22'6)'
Figure
(see
Campbell,
wild ancestors
.
if such changecould be achievedby artificial selection
that
reasoned
Darwin
in a relatively short period of tim-e,then natural selection should be capable
of
of consideruLt" *odincations of species over hundreds of thousands
generations.
334 UnitrV Mechanisms
of Evolution
'
Even if the advantagesof some heritable traits over others are slight, they
will accumulate in the population after many generations of natural
selection eliminating less favorable variations.
Gradualism is fundamentalto the Darwinian view of evolution. Darwin reasoned
that:
'
Life did not evolve suddenlyby quantum leaps, but instead by a gradual
accumulationof small changes.
'
Natural selection operating in differing contexts over vast spans of time
could account for the diversity of life.
SummarizingDarwin's view of evolution:
'
The diverse forms of life have arisen by descent with modification from
ancestralspecies.
'
The mechanism of modification has been natural selection workinc
gradually over long periods of time.
a. Some subtleties of natural selection
Populations are important in evolutionary theory, since a population is the
smallestunit that can evolve.
Population = A group of interbreedingindividuals belonging to a particular
speciesand sharing a common geographicarea
Natural selection is a consequenceof interactionsbetweenindividual organisms
and their environment, but individuals do not evolve.
'
Evolution can only be measuredas change in relative proportions of
variations in a population over several generations.
'
Natural selection can only amplifr or diminish heritable variations.
'
Organismscan adapt to changes in their immediate environment and
can be otherwise modified by life experiences, but these acquired
characteristicscannot be inherited.
'
Evolutionists must distinguishbetween adaptationsan organism acquires
during its lifetime and those inherited adaptations that evolve in a
population over many generationsas a result of natural selection.
Specifics of natural selectionare situational.
'
Environmental factors vary from area to area and from time to time.
'
An adaptationunder one set of conditions may be uselessor detrimental
in different circumstances.
b. Examples of natural selection in action
In an effort to test Darwin's hypothesisthat the beaks of Galapagosfinches are
evolutionary adaptationsto different food sources,Peter and Rosemary Grant
of Princeton University have been conducting a long-term study on medium
ground finches (Geospizaforlls) on Daphne Major, a tiny Galapagosisland.
They have discovered that:
'
Average beak depth (an inherited trait) oscillates with rainfall (see
Campbell,F igure 22.7).
+ In wet years, birds preferentially feed on small seeds,and
averagebeak depth decreases.
9 In dry years, small seeds are less plentiful, so survival
dependson the finches being able to crack the less preferred
larger seeds.Average beak depth increasesduring dry years.
View of Life 335
A Darwinian
With Modification:
22 Descent
Chapter
,
lt can be inferred that the change in beak depth is an adaptive response
to the relative availability of small seedsfrom year to year'
'n'1"u#,::;':::";,::;":,:;::i:#l"ffrYf
:T"lll"'"";,,."nmen
context may not work in another.
, Beak evolution on Daphne Maior does not result from inheritance of
acquired characterisiics. The environment did not create beaks
specializedfor large or small seeds, but only acted on inherited
variations already present in the population. The proportion of
finchesincreasedduring dry periodsbecause,on average,
thicker-beaked
thicker-beakedbirds transmitted their genesto more offspring than did
birds.
thinner-beaked
Michael Singerand Camille Parmesanof the University of Texas,have
evolutionaryadaptationin a butterfly population (Edith's
documented"rapid
checkerspot)livinginameadownearCarsonCity,Nevada.
.
In only a decade, this butterfly population ap_parently adapted to
changingvegetationby inheritedchangesin reproductivebehavior.
. Femaleslay eggs preferentially on certain plants which.provide food for
the larvae after they hatch. fn 1983, checkerspotslaid about 80% of
their eggs on a native plant, Collinsia parviflora'
.
By 1993, the butterflies were laying -about 70o/o.of their eggs on
Piantago lanceolata, an invading weed from surrounding cattle ranches'
. The researchersdemonstratedthat the switch in plant preference is
genetic; daughters of butterflies that deposited !9$s on Plantago
inherited the taste for that plant, choosing it over Collinsla when they
laid their eggs.
There are hundredsof examplesof natural selectionin laboratorypopulations
of such organismsas Drosophila. Other examplesof natural selection in action
include:
. Antibiotic resistancein bacteria(seeCampbell,Chapter 18)
.
Body size of guppies exposed to different predators (see campbell,
Chapter 1)
m.
Evidence of Evolution
A. Evidence of evolution pervades biology
an
Darwin used severallines of evidenceto supporthis principle of common descent,
lend
biology,
molecular
from
those
evolutionarychange.Recentdiscoveries,inciuding
supportto his evolutionaryview of life'
1. BiogeograPhY
It was biogeographicalevidencethat first suggestedcommon descentto Darwin,
b"cuure tn? Ul"oglographicalpatternshe observedonly made sensein the light of
e v o l u t i o n'
Biogeography:Thegeographicaldistributionofspecies
Islandshave many endemicspecieswhich are closely relatedto specieson the
nearestmainlandor neighboringisland.Somelogical questionsfollow:
336 unirIV Mechanisms
of Evolution
.
'
'
J
Why are two islandswith similar environments in different parts of the
world not populated by closely related species,but rather by species more
closely related to those from the nearest mainland even when that
environment is quite different?
Why are South American tropical animals more closely related to South
American desert animals than to African tropical animals?
Why does Australia have such a diversity of marsupial animals and very few
placental animals even though the environment can easily support
placentals?
The fossil record
Darwin was troubledby the absenceof transitionalfossilslinking modern life to
ancestralforms.
.
Even though the fossil record is still incomplete, paleontologistscontinue
to find importantnew fossils,and many key links are no longer missing.
.
For example, fossilized whales link these aquatic mammals to their
(seeCampbell,Figure 22.8).
terrestrialpredecessors
Although still incomplete,the fossil record provides informationthat supports
other types of evidence about the major branchesof the phylogenetic tree. For
example:
.
Prokaryotes are placed as the ancestorsof all life by evidence from cell
biology, biochemistry,and molecularbiology.
.
Fossilevidenceshowsthe chronological appearanceof the vertebratesas
being sequentialwith fishesfirst, followed by amphibians,reptiles and then
birds and mammals.This sequenceis also supportedby many other types of
evidence.
3 . Comparative anatomy
Anatomical similaritiesamong speciesgroupedin the sametaxonomiccategoryare
a reflection of their common descent.
.
The skeletalcomponentsof mammalianforelimbs are a good example (see
Campbell,Figure 22.9)
= Although the limbs are usedfor different functions, it is obvious that
the same skeletalelementsare present.
= It is logical that whetherthe forelimb is a foreleg,wing, flipper, or arm,
the basic similarity is the consequenceof descent from a common
ancestorand that the limbs have been modified for different functions.
They are homologous structures.
Homologous structures: Structuresthat are similar becauseof common ancestry
.
Other evidence from comparative anatomy supportsthat evolution is a
remodeling process in which ancestral structures that functioned in one
capacity have become modified as they take on new functions.
.
Some homologous structuresare vestigial organs.
Vestigial orgons : Rudimentary structuresof marginal or no use to an organism
.
Vestigial organsare remnants of structuresthat had important functions in
ancestralforms but are no longer essential.
'
Example: The remnants of pelvic and leg bones in snakesshow descent
from a walking ancestor,but have no function in the snake.
Modification:A DarwinianView of Life 337
Chapter22 DescentWith
Becauseit would be wasteful to continue providing blood, nutrients, and
spaceto structuresthat no longer have a major function, vestigial organs
serve evidenceof evolution by natural selection'
.
4. Comparative embrYologY
closely related organismsgo through similar stagesin their embryonic
development.
. Vertebrate embryos (fishes, amphibians, reptiles, birds, mammals) go
through an embryonic stage in which they possessgill slits on the sidesof
their throats(see Campbell,Figure 22.10).
. As developmentprogresses,
the gill slits develop into divergent structures
characteristicof each vertebrateclass.
.
In fish, the gill slits form gills; in humans, they form the eustachian tubes
that connect the middle ear with the throat.
Comparative embryology often establisheshomology among structures,such as gill
pou"'h.r, that becomeso alteredin later developmentthat their common origin is
not apparentby comparing their fully developed forms'
"ontogeny
In the late nineteenthcentury,embryologistsdevelopedthe view that
recapitulatesPhYlogenY'"
.
This view held that the embryonic developmentof an individual organism
(ontogeny) is a replay of the evolutionary history of the species
.
.
@hylogenY).
This is an extreme view; what does occur is a seriesof similar embryonic
stagesthat exhibit the same characteristics,not a sequenceof adult-like
stages.
OntoBeny can provide cluesto phylogeny, but all stagesof development
may becomemodified over the courseof evolution'
5. Molecular biologY
An organism'shereditary backgroundis reflected in its genesand their protein
products.
.
Siblingshave greater similarity in their DNA and proteins than do two
unrelatedorganismsof the same species.
.
Likewise, two speciesconsideredto be closely related by other criteria
shouldhave a greater proportion of their DNA and proteins in common
than more distantlyrelatedspecies.
Molecular taxonomistsuse a variety of modern techniquesto measurethe degree of
similarity among DNA nucleotidesequencesof different species.
.
The closer two speciesare taxonomically, the higher the percentage of
common DNA; this evidencesupportscommon descent'
.
Common descentis also supportedby the fact that closely related species
also have proteins of similar amino acid sequence(resultingfrom inherited
genes).
of monomers
. If two specieshave many genesand proteins with sequences
a common
from
copied
been
have
must
sequences
the
that maich closely,
ancestor.
Molecular biology has also substantiatedDarwin's idea that all forms of life are
relatedto someixtent through branchingdescentfrom the earliestorganisms(see
Campbell,Figure22.1l)'
338
Unit IV Mechanisms
of Evolution
Even taxonomically distant organisms(bacteria and mammals) have some
proteins in common.
For example, cytochrome c (a respiratory protein) is found in all aerobic
species.Cytochrome c moleculesof all speciesare very similar in structure
and function, even though mutations have substitutedamino acids in some
areas of the protein during the course of evolution.
Additional evidence for the unity of life is the common genetic code. This
mechanism has been passedthrough all branches of evolution since its
beginning in an early form of life.
B. What is theoretical about the Darwinian view of life?
Dismissing Darwinism as 'Just a theory" is flawed because:
.
Darwin madetwo claims:
l. Modern speciesevolved from ancestralforms.
2. The mechanism for evolution is natural selection.
'
The conclusion that specieschangeor evolve is basedon historical fact.
What, then, is theoretical about evolution?
.
Theories are conceptual frameworks with great explanatory power used to
interpret facts.
.
That speciescan evolve is fact, but the mechanism Darwin proposed for that
change-natural selection-is a theory. Darwin used this theory of natural
selection to explain facts of evolution documentedby fossils, biogeography,and
other historical evidence.
In science,"theory" is very different from the colloquial use of the word, which comes
closer to what scientistsmean by a hypothesis,or educatedguess.
'
Unifuing concepts do not become scientific theories, unless their predictions
stand up to thorough and continuoustesting by experiment and observation.
.
Good scientists, however, do not allow theories to become dogma; many
evolutionary biologists now question whether natural selection alone can
account for evolutionarv historv observedin the fossil record.
REFBRENCES
Campbell,N., et al. Biology.5th ed. Menlo Park, California:Benjamin/Cummings,1998.
Futuyma, D.J. Evolutionary Biolog,,.2nd ed. Sunderland,Massachusetts:Sinauer,1986. Because it
is broad in scope,this advancedtextbook is a good referencefor backgroundinformation.
Mayr, E. The Growth of Biological Thought: Diversity, Evolution and Inheritance. Cambridge,
Massachusetts:Harvard University Press, 1982. This is an excellent reference for historical
perspective and could be a useful companion to the text for supplementing lecture material on
many topics.